Rashi on Deuteronomy 32:18/רש"י על דברים ל״ב:י״ח

1תשי means THOU HAST FORGOTTEN. But our Rabbis explained it thus: whenever He was about to bestow good upon you, you provoked Him to anger and, as it were, weakened (תש) His power so that He could not do you good (Sifrei Devarim 319:3).

2אל מחללך means, God Who has brought you forth out of the womb. It has the same meaning as in (Psalms 29:9) “[The voice of the Lord] makes hinds to bring forth (יחולל)”, and (Psalms 48:7) “travail (חיל) as that of a woman in childbirth”.